Process for manufacturing sheet of austenitic stainless steel having high mechanical properties and sheet thus obtained

ABSTRACT

The invention relates to a hot-rolled sheet made of austenitic stainless steel, the chemical composition of which comprises, the contents being expressed by weight: 0.015%≦C≦0.030%, 0.5%≦Mn≦2%, Si≦2%, 16.5%≦Cr≦18%, 6%≦Ni≦7%, S≦0.015%, P≦0.045%, Al≦0.050%, 0.15%≦Nb≦0.31%, 0.12%≦N≦0.16%, the Nb and N contents being such that: Nb/8+0.1%≦N≦Nb/8+0.12%, optionally: Mo≦0.6%, 0.0005%≦B≦0.0025%, the balance of the composition consisting of iron and inevitable impurities resulting from the smelting.

The present invention relates to the manufacture of hot-rolled sheetmade of austenitic stainless steel having high mechanical properties andespecially a very advantageous combination of mechanical strength anduniform elongation.

For the manufacture of structural components in the automotive industry,it is common practice to use various grades of coated carbon steelsheets having more or less complex microstructures. The parts areproduced from sheets having a thickness ranging from 1 to 3 mm. However,for some parts it would be desirable to have both a higher corrosionresistance combined with a high deformability so as to produce partswith a complex drawing operation. Moreover, it is known that austeniticstainless steels are widely used because of their excellent corrosionresistance and their mechanical properties, in particular their highductility. For example, austenitic stainless steels denoted according tothe EN 10088-1 standard by the reference 1.4318 are known in which thecomposition contains (in contents expressed by weight): C≦0.030%,Si≦1.00%, Mn≦2.00%, P≦0.045%, S≦0.015%, Cr: 16.50 to 18.50%, Ni: 6.00 to8.00%, N: 0.10 to 0.20%. These steels have high mechanical propertiesowing to the formation of martensite during deformation at roomtemperature. Typical mechanical properties of these steels in theannealed state are the following: yield strength R_(p0.2) (conventionalyield strength corresponding to a 0.2% strain): 300-400 MPa; uniformelongation: A≧45%, R_(m) (maximum strength)≧700 MPa; product P=R_(p0.2)(MPa)×uniform elongation=about 15750 MPa. %. It is possible to use thesegrades in the state work-hardened by cold rolling: C850,C1000-EN-10088-2 standard, these designations corresponding to a minimumstrength of 850 and 1000 MPa respectively. The increase in yieldstrength conferred by this operation (R_(p0.2)≧600 MPa) is manifested bya simultaneous reduction in elongation (A=30%). The product P thenreaches about 18000 MPa. %. These properties are satisfactory forcertain applications. However, they remain insufficient if high strengthin service is desired, for example for an increase in lightening, and ahigh capability for prior forming operations.

An alternative method to work hardening by cold rolling is workhardening by hot rolling at a sufficiently low temperature. This methodgives a better elongation-strength compromise, but has the majordrawback of leading to local deformations during forming, resulting invermicular defects. To avoid these vermicular defects on a standard1.4318 steel not recrystallized after hot rolling, it is necessary tocarry out an annealing operation after the hot rolling.

The object of the invention is therefore to provide hot-rolled sheets ofaustenitic stainless steel having mechanical properties superior orequivalent to those of grades of the 1.4318 type mentioned above, whichare inexpensive to manufacture and are insensitive to the appearance ofvermicular defects.

The object of the invention is also to provide hot-rolled sheets made ofaustenitic stainless steel having a product P greater than 21000 MPa. %,which may be combined with a yield strength R_(p0.2) of greater than 650MPa, or else of a uniform elongation of greater than 45%.

For this purpose, the subject of the invention is a hot-rolled sheetmade of austenitic stainless steel, the product P (R_(p0.2)(MPa)×uniform elongation (%) of which is greater than 21000 MPa. % andthe chemical composition of which comprises, the contents beingexpressed by weight: 0.015%≦C≦0.030%, 0.5%≦Mn≦2%, Si≦2%, 16.5%≦Cr≦18%,6%≦Ni≦7%, S≦0.015%, P≦0.045%, Al≦0.050%, 0.15%≦Nb≦0.31%, 0.12%≦N≦0.16%,the Nb and N contents being such that:

Nb/8+0.1%≦N≦Nb/8+0.12%, optionally: 0.0005%≦B≦0.0025%, Mo≦0.6%, thebalance of the composition consisting of iron and inevitable impuritiesresulting from the smelting.

According to a preferred embodiment, the niobium and nitrogen contentsof the steel, expressed by weight, are such that: 0.20%≦Nb≦0.31%,0.12%≦N≦0.16%.

The subject of the invention is also a hot-rolled sheet made ofaustenitic stainless steel according to any one of the abovecompositions, the yield strength R_(p0.2) of which is greater than 650MPa, characterized in that the mean austenitic grain size of the steelis less than 6 microns, in that the non-recrystallized surface fractionis between 30 and 70% and in that the niobium is completely in the formof precipitates.

The subject of the invention is also hot-rolled sheet made of austeniticstainless steel according to any one of the above features, the uniformelongation of which is greater than 45%, characterized in that theniobium is not completely precipitated.

The subject of the invention is also a process for manufacturing ahot-rolled sheet made of austenitic stainless steel, the yield strengthR_(p0.2) of which is greater than 650 MPa, in which: a semi-finishedproduct made of steel having the composition according to any one of theabove compositions is supplied; then said semi-finished product isreheated to a temperature of between 1250° C. and 1320° C.; and thensaid semi-finished product is rolled with an end-of-rolling temperaturebelow 990° C. and a cumulative reduction ratio E on the last twofinishing stands of greater than 30%.

According to one particular embodiment, a semi-finished product made ofsteel having the composition above, containing 0.20%≦Nb≦0.31%,0.12%≦N≦0.16%, is supplied and then said semi-finished product is rolledwith an end-of-rolling temperature below 970° C.

The subject of the invention is also a process for manufacturing ahot-rolled sheet made of austenitic stainless steel, the uniformelongation of which is greater than 45%, in which: a semi-finishedproduct made of steel having the composition according to any one of theabove compositions is supplied; then said semi-finished product isreheated to a temperature of between 1250° C. and 1320° C.; and thensaid semi-finished product is rolled with an end-of-rolling temperatureabove 1000° C.

The subject of the invention is also a process for manufacturing ahot-rolled sheet made of austenitic stainless steel, the product P(R_(p0.2) (MPa)×uniform elongation (%)) of which is greater than 21000MPa. %, in which: a semi-finished product made of a steel having thecomposition according to any one of the above compositions is supplied;then said semi-finished product is reheated to a temperature of between1250° C. and 1320° C.; and then said semi-finished product ishot-rolled.

The subject of the invention is also the use of a hot-roiled sheet madeof stainless steel according to any one of the above features ormanufactured by any one of the above processes, for the manufacture ofstructural components in the automotive field.

Other features and advantages of the invention will become apparent overthe course of the description below given by way of example.

After many trials, the inventors have shown that the variousrequirements mentioned above are satisfied by observing the followingconditions:

As regards the chemical composition of the steel, the carbon contentmust be equal to or less than 0.030% so as to avoid the risk ofsensitivity to intergranular corrosion. For the purpose of obtaining ayield strength of greater than 650 MPa, the carbon content must be equalto or greater than 0.015%.

Manganese, like silicon, is an element known for its deoxidizingproperties in its liquid state and for increasing the hot ductility, inparticular by being combined with sulphur. Moreover, at ambienttemperature, manganese promotes stability of the austenitic phase andreduces the stacking fault energy. It also increases the solubility ofnitrogen. These favourable effects are obtained inexpensively when themanganese content is between 0.5 and 2%.

Like manganese, silicon is an element usually added for the purpose ofdeoxidizing the liquid steel. Silicon also increases the yield strengthand the tensile strength, by solid-solution hardening or by its actionon the content of ferrite δ. However, above 2%, the weldability and hotductility are reduced. Chromium is an element well known for increasingthe oxidation resistance and corrosion resistance in aqueous medium.This effect is obtained satisfactorily when its content is between 16.5%and 18%.

Nickel is an essential element for ensuring sufficient stability of theaustenitic structure of the steel at ambient temperature. The optimumcontent must be determined in relation to other elements of thecomposition promoting alpha-phase formation, such as chromium, or thosepromoting gamma-phase formation, such as carbon and nitrogen. Its effecton the stability of the structure is sufficient when its content isequal to or greater than 6%. Above 7%, the production cost increasesexcessively because of the expense of this addition element.

Molybdenum enables the pitting resistance to be increased. Optionally,an addition of molybdenum in an amount ranging up to 0.6% may be carriedout. Boron is used to improve the forgibility of the steel. Optionally,an addition of boron in an amount of between 0.0005 and 0.0025% may becarried out. An addition with a greater amount would critically reducethe burning temperature.

Sulphur is an element that particularly degrades the hot forgibility andthe corrosion resistance—its content must be kept equal to or less than0.015%. Phosphorus likewise degrades the hot ductility—its content mustless than 0.045% in order to obtain satisfactory results.

Aluminium is a powerful agent for deoxidizing the liquid metal. Incombination with the abovementioned silicon and manganese contents, anoptimum effect is obtained when its content is equal to or less than0.050%.

Niobium and nitrogen are important elements of the invention for thepurpose of manufacturing austenitic stainless steels having highmechanical properties.

Niobium retards recrystallization during hot rolling—for a givenend-of-hot-rolling temperature, its addition results in a higherwork-hardening factor being maintained (the hot rolling is said to be“work hardening”), thus increasing the tensile strength of this steel.It is generally used like Ti to combat the formation of chromiumcarbides (EN 1.4580 and EN 1.4550 Nb stabilized austenitic stainlesssteels). Finally, it may lead to the formation of an intermetallic phasegiving an improvement in hot creep resistance.

Nitrogen is an element hardening in interstitial solid solution, whichmost particularly increases the yield strength in this regard. It isalso known, in solid solution, as a powerful stabilizer for theaustenitic phase and as a retarder for the precipitation of chromiumcarbides Cr₂₃C₆. The solubility of nitrogen during solidification goesthrough a maximum—too high a content results in the formation of volumedefects in the metal.

The combined addition of niobium and nitrogen for the purpose ofhardening is somewhat unusual in austenitic stainless steels. Within thecontext of the invention, it has been demonstrated that stainless steelshaving a composition close to that of the abovementioned 1.4318 steelsadvantageously benefit from a particular combined addition of niobiumand nitrogen, optimized for the purpose of obtaining certain mechanicalproperties under precise conditions, that are mentioned below:

Firstly, it has been demonstrated that a nitrogen content ranging from0.12 to 0.16%, together with a niobium content ranging from 0.15 to0.31%, the niobium and nitrogen contents being such that:Nb/8+0.1%≦N≦Nb/8+0.12% (relationship 1), make it possible to manufacturea hot-rolled sheet having high mechanical properties intended to bedrawn, without the need for annealing after rolling as in conventional1.4318 steels, the drawn part not being subject to the formation ofvermicular defects.

The precipitation of nitrides NbN, which occurs during the end of hotrolling, reduces the amount of nitrogen in solid solution. The aboverelationship (1) keeps as much nitrogen in solid solution, aftercomplete precipitation of all the available niobium, as in the 1.4318grade (N≧0.1%).

This therefore makes it possible to obtain the same metastability of theaustenite at ambient temperature. The possibility of reducing the Nicontent by increasing the N content is limited by the solubility limitof nitrogen in the steel during solidification. For the Cr, Mn and Nicontents of the steel according to the invention, the nitrogen contentmust be equal to or less than 0.16%.

A sufficient amount of niobium must be present so as to obtain ahardening effect and to retard the recrystallization. This amount mustbe adapted so as to obtain an NbN solvus above the end-of-rollingtemperature in order to obtain precipitation at the end of hot rolling.

The niobium and nitrogen contents according to the invention enablesubstantial precipitation of NbN after hot rolling to be obtained.

A combined addition of 0.15 to 0.31% niobium (preferably 0.20 to 0.31%niobium) and 0.12 to 0.16% nitrogen, the niobium and nitrogen contentsbeing such that: Nb/8+0.1%≦N≦Nb/8+0.12%, makes it possible to obtain anadvantageous yield strength/elongation combination, the product P ofwhich is greater than 21000 MPa. %.

Apart from iron, the remainder of the composition consists of inevitableimpurities resulting from the smelting, such as for example Sn or Pb.

The manufacturing process according to the invention is implemented asfollows:

A steel having a composition explained above is smelted. This smeltingmay be followed by the steel being cast into ingots or, in the mostgeneral case, cast continuously, for example in the form of slabsranging from 150 to 250 mm in thickness. The casting may also be carriedout in the form of thin slabs a few tens of millimetres in thicknessbetween steel counter rotating rolls. These cast semi-finished productsare firstly heated to a temperature between 1250 and 1320° C. Thepurpose of the 1250° C. temperature is to dissolve any niobium-basedprecipitates (nitrides and carbonitrides). However, the temperature mustbe below 1320° C. for fear of being too close to the solidus temperaturewhich could be reached in certain segregated zones and of causing alocal onset of a liquid state that would be deleterious to hot forming.In the case of direct casting of thin slabs between counter rotatingrolls, the step of hot-rolling these semi-finished products starting ata temperature below 1250° C. may take place directly after casting sothat an intermediate reheat step is unnecessary in this case.

The rolling is generally carried out on a continuous hot-rolling millcomprising in particular roughing stands and finishing stands. It hasbeen demonstrated that a particularly high yield strength of R_(p0.2) isobtained by especially controlling the reduction ratio in the last twofinishing stands: if the thickness of the sheet entering the penultimatefinishing stand is denoted by e_(N-2) and the thickness of the sheetexiting the last finishing stand is denoted by e_(N), the cumulativereduction ratio over the last two finishing stands is defined by:

$ɛ = {\frac{e_{N - 2} - e_{N}}{e_{N - 2}}.}$

According to the invention, it has been demonstrated that when theend-of-rolling temperature is below 990° C. and when the cumulativereduction ratio ε is greater than 30%, the yield strength R_(p0.2) ofthe final product obtained is greater than 650 MPa, the niobium thenbeing completely in the form of precipitates.

For a Nb content of between 0.20 and 0.31% and a nitrogen contentbetween 0.12 and 0.16%, this 650 MPa minimum value is obtained when theend-of-rolling temperature is below 970° C. and ε is greater than 30%.

According to the invention, it has also been demonstrated that it ispossible to obtain a hot-rolled sheet with a uniform elongation ofgreater than 45% when the end-of-rolling temperature is above 1000° C.In this case, the niobium is partially precipitated.

After hot rolling, a sheet is obtained that is not sensitive to theappearance of vermicular defects and does not require intermediateannealing.

As a non-limiting example, the following results will show theadvantageous characteristics conferred by the invention.

EXAMPLE

Semi-finished products were produced by casting steel having thecomposition presented in the table below (in wt %):

TABLE 1 Composition of the steels (in wt %) Steel C Mn Si Cr Ni Mo S PAl Nb N I1 0.023 1.100 0.48 17.45 6.67 0.25 0.005 0.020 0.002 0.152 0.13(according to the invention) I2 0.024 1.19 0.55 17.36 6.66 0.25 0.0050.020 0.002 0.302 0.15 (according to the invention) R 0.026 1.030 0.617.5 6.6 0.25 0.0008 0.026 0.002 0.002 0.13 (reference) Underlinedvalues: not according to the invention

The semi-finished steel products were reheated at 1280° C. for 30minutes. A hot-rolling operation was then carried out by varying theend-of-rolling temperature between 900 and 1100° C. and the cumulativereduction ratio ε, so as to reach a final thickness of 3 mm. Steelsheets I1-1, I1-2, I1-3, etc. denote sheets obtained from the samesemi-finished product I1 rolled under different conditions. Themicrostructure of the steel obtained was characterized by measuring inparticular the surface fraction of recrystallized austenitic phase, thefraction of precipitated niobium relative to the total niobium and theaverage grain size. In the case of an incompletely recrystallizedstructure, the latter measurement was carried out on the recrystallizedpart of the structure. The tensile mechanical properties were alsodetermined, in particular the yield strength R_(p0.2) and the uniformelongation. The possible presence of local deformation during thetensile trial was also recorded. It is known that the presence of such alocal deformation is associated with the appearance of vermiculardefects during forming operations.

The results are given in Table 2 below:

TABLE 2 Manufacturing conditions, microstructural characteristics andmechanical properties of hot-rolled sheets Average Non- grain sizerecrystallized Niobium Trial EOR less than fraction between completelyR_(p0.2) A R_(p0.2) × A Localized No. (° C.) ε > 30% 6 microns 30 and70% precipitated (MPa) (%) (MPa. %) deformation I1-1 905 Yes Yes Yes Yes689 40 27628 No I1-2 935 Yes Yes Yes Yes 651 40 25520 No I1-3 1040 YesNo No (<30%) No 432 49 21340 No I1-4 1050 Yes No No (<30%) No 467 4621715 No I2-1 930 Yes Yes Yes Yes 677 38 25997 No I2-2 965 Yes Yes YesYes 681 39 26559 No I2-3 980 No No Yes Yes 631 41 26186 No I2-4 1000 NoYes No (<30%) No 627 46 28277 No I2-5 1100 Yes No No (<30%) No 547 5329100 No R-1 900 Yes — Yes — 702 29 20428 Yes R-2 925 Yes — Yes — 638 2918566 Yes R-3 950 Yes — Yes — 632 30 19150 Yes R-4 1020 Yes — No (<30%)— 482 31 14749 No EOR: End-of-rolling temperature; R_(p0.2:):Conventional yield strength at 0.2% strain; A: Uniform elongation; ε:Cumulative reduction ratio of the last two rolling passes.

Thus, the above table shows that steels I1 and I2 according to theinvention have a particularly advantageous product R_(p0.2)×A of greaterthan 21000 MPa. %, whereas the reference R steel does not have such aproduct, irrespective of the rolling conditions.

This table also shows that, when the non-recrystallized fraction isbetween 30 and 70% and when the average grain size is less than 6microns, the yield strength R_(p0.2) is greater than 650 MPa (trialsI1-1, I1-2, I2-1, I2-2). Moreover, when the non-recrystallized fractionis greater than 70%, the elongation tends to be reduced.

These properties are obtained for steels having a niobium content ofbetween 0.15 and 0.31%, and a nitrogen content of between 0.12 and0.16%, the niobium and nitrogen contents being such that:Nb/8+0.1%≦N≦Nb/8+0.12%, the end-of-rolling temperature being below 990°C. and the cumulative reduction ratio ε being greater than 30%.

In the case of steels having a niobium content of between 0.20% and0.31% and a nitrogen content of between 0.12 and 0.16%, the niobium andnitrogen contents being such that: Nb/8+0.1%≦N≦Nb/8+0.12%, theseproperties are obtained when the end-of-rolling temperature is below970° C. and when the cumulative reduction ratio ε is greater than 30%(trials I2-1 and I2-2).

When the niobium is not completely precipitated (trials I1-3, I1-4,I2-4, I2-5), the uniform elongation is greater than 45%. For the steelcompositions according to the invention, this result is obtained whenthe end-of-rolling temperature is above 1000° C. For comparison, thereference steel does not offer such properties.

Therefore, certain manufacturing conditions (end-of-rolling temperatureand cumulative reduction ratio) will be more particularly chosendepending on whether it is desired to produce a steel sheet having aparticularly high yield strength or instead one having a high elongationcapability.

Moreover, the stress-strain curves of the steels according to theinvention show no plateau indicating local deformation, whatever thehot-rolling conditions, in contrast with the reference steel thatexhibits local deformation whenever it is partially recrystallized(trials R-1, R-2, R-3). This point is particularly advantageous for theforming operation, by ensuring that there are no vermicular defects.

Thus, because of their particularly high mechanical properties, andespecially their very advantageous yield strength×uniform elongationproduct, the hot-rolled steel sheets according to the invention will beadvantageously used for applications requiring good formability and highcorrosion resistance. When they are used in the automotive industry,their advantages will be profitably enjoyed for the economic manufactureof structural components.

1. A hot-rolled sheet comprising austenitic stainless steel, the productP (R_(p0.2) (MPa)×uniform elongation (%)) of which is greater than 21000MPa. % and the chemical composition of which comprises, the contentsbeing expressed by weight: 0.015%≦C≦0.030% 0.5%≦Mn≦2% Si≦2% 16.5%≦Cr≦18%6%≦Ni≦7% S≦0.015% P≦0.045% Al≦0.050% 0.15%≦Nb≦0.31% 0.12%≦N≦0.16% the Nband N contents being such that: Nb/8+0.1%≦N≦Nb/8+0.12% optionally:0.0005%≦B≦0.0025% Mo≦0.6%, the balance of the composition consistingessentially of iron and inevitable impurities resulting from thesmelting.
 2. The hot-rolled sheet according to claim 1, wherein theniobium and nitrogen contents of said steel, expressed by weight, aresuch that: 0.20%≦Nb≦0.31% 0.12%≦N≦0.16%.
 3. The hot-rolled sheetaccording to claim 1, having a yield strength R_(p0.2) greater than 650MPa, wherein the mean austenitic grain size of said steel is less than 6microns, the non-recrystallized surface fraction is between 30 and 70%and the niobium is completely in the form of precipitates.
 4. Thehot-rolled sheet according to claim 1, having a uniform elongationgreater than 45%, wherein the niobium is not completely precipitated. 5.A process for manufacturing a hot-rolled sheet comprising austeniticstainless steel, the yield strength R_(p0.2) of which is greater than650 MPa, comprising: heating a semi-finished product comprising steelhaving the composition according to claim 1 to a temperature of between1250° C. and 1320° C.; and then rolling said semi-finished product withan end-of-rolling temperature below 990° C. and a cumulative reductionratio ε on the last two finishing stands of greater than 30%.
 6. Themanufacturing process according to claim 5, wherein the steel of thesemi-finished product comprising steel comprises niobium and nitrogen inthe following amounts, expressed by weight: 0.20%≦Nb≦0.31%0.12%≦N≦0.16%, and said semi-finished product is rolled with anend-of-rolling temperature below 970° C.
 7. A process for manufacturinga hot-rolled sheet comprising austenitic stainless steel, the uniformelongation of which is greater than 45%, comprising: heating asemi-finished product comprising steel having the composition accordingto claim 1 to a temperature of between 1250° C. and 1320° C.; and thenrolling said semi-finished product with an end-of-rolling temperatureabove 1000° C.
 8. A process for manufacturing a hot-rolled sheetcomprising austenitic stainless steel, the product P (R_(p0.2)(MPa)×uniform elongation (%)) of which is greater than 21000 MPa. %,comprising: heating a semi-finished product made of a steel having thecomposition according to claim 1 to a temperature of between 1250° C.and 1320° C.; and then hot-rolling said semi-finished product. 9.(canceled)
 10. The hot-rolled sheet of claim 1, the balance of thecomposition consisting of iron and inevitable impurities resulting fromthe smelting.